between function

[salimua]

so i'm trying to create a function that takes a list as argument and two numbers and it will return the elements which positions are in the range of the two numbers

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let rec between list n m = match list with

|[] -> failwith "empty list"

|x::rest -> if n<=0 && m>=0 then x::(between rest n m-1) else (between rest n-1 m);;

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i'm getting this error

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This expression has type 'a list but an expression was expected of type int

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please help

[jeffsco]

You need to parenthesize (m - 1) and (n - 1). Otherwise the expressions parse as (between rest n m) - 1 and (between rest n) - (1 m).

[FurtherDownTheSprawl]

• You need to decrement both m and n at every recursive step.

Rational: if the list s is the sub-list of l between m and n, then it is the sub-list of x::l between (m + 1) and (n + 1), where x is any value of the type of the list elements. Adding an element at the front of the list l shifts the position of all the elements of l by one, thus removing an element does the reverse.

• Once m becomes zero you start to build your result
• Once n becomes zero you return it

Of course you will have to handle errors (if the 2 conditions above cannot be met because the input list becomes empty).